Molecular Dynamics Investigation on Coke Ash Behavior in the High

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A molecular dynamics investigation on coke ash behaviour in the high temperature zones of a blast furnace: Influence of alkalis Kejiang Li, Rita Khanna, Jianliang Zhang, Mohammed Bouhadja, Minmin Sun, Mansoor Barati, Zhengjian Liu, and Chandra Veer Singh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02795 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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A molecular dynamics investigation on coke ash behavior in the high temperature zones of a blast furnace: Influence of alkalis

Kejiang Li

a, *

, Rita Khanna b, Jianliang Zhang a, Mohammed Bouhadja c, Minmin Sun a,

Mansoor Barati d, Zhengjian Liu a, and Chandra Veer Singh d

a

School of Metallurgical and Ecological Engineering, University of Science and

Technology Beijing, Beijing 100083, P.R. China. b

School of Materials Science and Engineering, The University of New South Wales,

Sydney, NSW 2052 Australia. c

Department of Physics, The University of Picardy Jules Verne, Amiens 80000, France.

d

Department of Materials Science and Engineering, University of Toronto, Toronto, ON

M5S 3E4, Canada.

*

Corresponding author: Dr. Kejiang Li

E-mail: [email protected] Phone: +86-010-62332550 Address: 30 Xueyuan Road, Haidian District, Beijing, 100083, P. R. China

1 ACS Paragon Plus Environment

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ABSTRACT With specific focus on local structural order, bonding networks, transport properties and viscosity of the molten ash oxides, we report molecular dynamics simulations on the influence of alkalis (Na2O and K2O) on coke behavior within a blast furnace. Atomistic simulations were carried out on the Al2O3-SiO2-CaO-K2O-Na2O system at 2223 K for a range and relative proportions of Na2O and K2O. Alkalis were seen to have a strong effect on the oxygen bonding networks; the relative proportions of bridging and non-bridging oxygen showed a sharp increase while significant reductions were observed for tri-cluster oxygens. Total diffusion coefficients and viscosity showed a highly non-linear dependence on the relative proportions of two alkalis with large changes observed in the simultaneous presence of alkalis as compared to their individual presence. Our studies have shown that the combined influence of alkalis on the viscosity of molten ash, and associated coke degradation within a blast furnace is likely to be much smaller than previously perceived, and could even be negligible for some alkali concentrations.

Key Words: Blast furnace; alkalis; coke ash; aluminosilicates; molecular dynamics


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1. Introduction While facing stiff economic and environmental challenges with continuous improvements, the blast furnace continues to be the primary production method of hot metal in large scale steel production. The global steelmaking capacity is expected to increase to 1.6 billion tonnes by 2017, with most of the growth taking place in Asia based on integrated steelmaking projects involving blast furnace (BF) and basic oxygen steelmaking (BOF) technologies 1. Consistent efforts are however being made towards reducing coke consumption in BF ironmaking in attempts to reduce greenhouse gas emissions and energy consumption. Coke plays a significant role in several key aspects of BF operations, e.g., as a source of reducing gases, supplying heat/energy, providing carbon for carburizing hot metal, and supporting the weight of the burden 2. Following injection into a blast furnace, coke traverses down various thermal zones of a blast furnace and undergoes several reactions and transformations. Upon exposure to temperatures above 1073 K, the carbon network in coke starts to gasify and mineral impurities (ash oxides) start drifting out of the carbon matrix as slag, impacting coke strength, rigidity and reactivity. The detrimental influence of molten coke ash is further exacerbated by the increasing use of poor quality raw materials, a current trend caused by resource shortages and dwindling supplies of high quality coking coals 3. To minimize the influence of ash impurities on key coke characteristics, it is significantly important to develop a fundamental understanding of factors affecting the structural evolution, melting behavior, and the fluidity of molten coke ash.


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The composition of coke ash can be highly variable depending on the impurity contents of raw coals used in the coking process, and/or in-situ reduction of ash oxides (e.g., SiO2, Fe2O3, CaO, MnO etc.) within a blast furnace. During the coking process (1373 K), primary aluminosilicates (kaolinite, quartz, illite, muscovite etc.) present initially in coking coals are known to transform into quartz, cristobalite and mullite etc.

4, 5

. Significant

progress has been made in understanding the role of major constituents (SiO2, Al2O3, CaO) in aluminosilicate glasses and networks; extensive investigations have been carried out on several aspects: non-framework ion effects and aluminum location

6

; degree of

polymerization in glasses and melts 7; ionic conductivity and melt viscosity 8; classical MD and quantum cluster studies on oxygen tri-clusters 9; role of non-bridging oxygen on glass transition temperature 10; the influence of silica reduction etc.11 Studies on ash behavior/and aluminosilicates in coals and cokes have focused on mineral reactions5, critical viscosity and ash fusion temperatures12, flow behavior of slags from coal and coke13, slag viscosity 14 etc. The high temperature behavior of molten aluminosilicates is strongly influenced by the local atomic configuration of its constituents and associated short/long range structural order. These fluid oxides can however form a slag layer on the surface of coke particles creating a barrier for further reactions of coke with gas and liquids15, 16. However, the influence of alkalis on various properties of aluminosilicates is not fully understood. Alkali oxides (K2O and Na2O), even when present in small concentrations, are known to significantly affect the properties of molten aluminosilicate slags 17, 18. A certain amount of alkalis, mainly in the form of complex alkali-silicates or alkali-aluminosilicates compounds such as orthoclase (K2Al2Si6O16), albite (Na2Al2Si6O16), leucite (K2Al2Si4O12), kaliophilite (K2Al2Si2O8), etc. are charged into blast furnace along with the burden


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materials4. These undergo disintegration and get reduced to alkali metal vapors during exposure to high temperatures in the lower regions of the blast furnace 5. The ascending alkali vapors get re-oxidized and subsequently condensed in the upper regions (low temperature) of the blast furnace. Through continuous evaporation-condensation-circulation cycles, concentrations of alkalis can build to high levels in different regions of the blast furnace. Jiao et al.

19

have shown that the concentration of K and Na in the belly of the

blast furnace can reach up to 50 times of their initial concentration in the charge. These alkalis are known to have a detrimental influence on the furnace performance, the hot strength of coke, refractory lining, scaffolding, fuel consumption etc. 20. The incorporation of alkalis in BF cokes at 1573 K was experimentally observed to lead to the formation of kalsilite, potassium or sodium aluminum silicates causing lattice expansion, crack generation and catalytic gasification of carbon 21. Both alkalis (present in various forms depending on their location in the BF) are usually present simultaneously in the blast furnace with their relative proportions depending on the charged feedstock, furnace size and the location within. Dissection of a water quenched commercial BF (128 m3 inner volume) in China showed that the concentrations of both Na and K started to increase significantly in the bosh area reaching a maximum in the belly; the concentration of alkalis near the furnace edges was nearly twice the corresponding concentrations in the central regions of BF

19

. Kurumov et al

20

have

reported on the average monthly alkalis balance in a 3200 m3 blast furnace and recorded a continuous simultaneous presence of both K2O and Na2O during BF operations. Our understanding of alkali behaviour within a blast furnace is currently very limited, as most of the studies reported in the literature have focused their attention on individual alkalis and


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therefore do not represent the real situation within an operating BF 22-26. To the best of our knowledge, no theoretical or experimental investigation has reported on coke behaviour in the simultaneous presence of both K2O and Na2O. This major gap in our knowledge base is the focus of our investigation. This study aims to develop a fundamental understanding of the combined effect of K2O and Na2O on the overall structure, fluidity and viscosity of molten coke ash. An in-depth molecular dynamics simulation was carried out on the Al2O3SiO2-CaO-K2O-Na2O system to investigate the joint role of alkalis on the local structural order, bonding networks and transport properties of individual ions along with implications for the overall fluidity of the molten oxide system.

2. Simulation approach Relative proportions of constituent oxides in the Al2O3-SiO2-CaO system were chosen based on ash compositions in cokes recovered from various blast furnace dissections 2, and on their reduction reactions in the high temperature zones of blast furnace

22, 27

. The total

concentration of both alkalis (Na2O+K2O) was varied from 0 to 10% to reflect their concentration profile within an operating BF; their relative proportion in the mix was varied as follows: K2O/Na2O = 1/3, 1/2, 1/1, 2/1 to 3/1. The label K1N3 represents K2O/Na2O mass ratio of 1:3. Studies were also carried out on 100% K2O (K1) and 100% Na2O(N1) systems for sake of comparison. Various chemical compositions of aluminosilicates used in these simulations are given in the Table S1 along with melting points of selected melt systems (estimated using FactSage™ 7.0

28

). A total of 10000 atoms were used in MD

simulations. The molar masses and densities of various atoms were used to determine the


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size of the cubic simulation box. The simulation temperature was chosen as 2223 K, which was higher than the melting points of various constituents. As this temperature is slightly lower than the highest temperature experienced by coke in the tuyere zone of a blast furnace (> 2373 K) 3, it represents the temperature experienced by coke while traversing down the blast furnace. Simulations were carried out in an NVT ensemble with periodic boundary conditions, keeping the number of particles (N), system volume (V) and temperature (T) fixed during the simulations. Densities of selected melts at 2223 K and standard atmospheric pressure were calculated by dividing the total mass by the total volume of the melt, as obtained by summing partial molar volumes of constituent oxides. Partial molar volumes of various oxides were calculated using the coefficients of thermal expansion and isothermal compressibility of each oxide. Details of computation method and parameters used can be found elsewhere26. The densities for various simulated systems have been summarized in Table S2. Well-known Miyake potential was used in this investigation for interactions between various ion pairs29. This potential can be written as:

U (rij ) =

zi z j e2 rij

{

}

 ai + a j − rij  cic j − a ⋅( r −r ) 2 + f0 (bi + bj )exp  − 6 + Dij 1− e ij 0  −1   b +b  r  i j  ij

(1)

where rij represents the interatomic distance between ions i and j; f0 (=6.9511×10-11 N) an empirical constant; e the effective electronic charge; and z, a, b, c represents parameters for various ionic species; Dij, aij, r0 are the parameters between cations-oxygen pairs. The first term in Eq. 1 represents the long-range Coulomb potential and the second term represents the repulsive Born interaction term from electronic overlap as two ions approach each other.


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The third term is the attractive van der Waals term due to dipole-dipole interactions and the fourth term is the short-range Morse function. All potential parameters have been provided in Table 1. The transferability of Miyake potential for the Al2O3-SiO2-CaO-K2O-Na2O system was validated in a recent study by comparing the simulated structural properties and viscosity of the oxide system with experimental results.19 Using LAMMPS package [version 16 Feb 2016], MD simulations were performed with an integration time-step of 1 fs 30. Long range Coulomb interactions were evaluated by the Ewald summation method with a cutoff distance of 10 Å; the corresponding cutoff distances for the short-range Born &Van der Waals, and Morse interactions were chosen to be 8.0 Å and 5.5 Å respectively. The simulations were started at T = 5000 K from a random configuration of different atoms; the system was thermalized for 100 ps and then cooled down to 2223 K in 277.7 ps with a cooling rate of 1013 K/s. The quenching rate needs to be chosen carefully as very high quenching rates can hinder lattice relaxation, and may also freeze some local structural features. The system was then kept at 2223 K for 500 ps to achieve equilibrium. Several iteration loops were carried out to ensure equilibrium till no further variation was observed. The mean square displacement of atoms was monitored regularly to ensure the molten state of the simulated system. In the final stage, the system was equilibrated at 2223 K for 1000 ps and the simulation data was collected. Simulation trajectories were analyzed using the ISSACS program 31 to determine coordination numbers and cluster statistics. The simulation box was divided into 40 slabs of equal thickness for reverse non-equilibrium molecular dynamics computations (RNEMD)

32

. The momentum

was exchanged every five steps. After a series of trial runs, 1000 ps runs were carried out


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for a steady linear velocity profile. Additional details on computing various system properties have been provided in the Supplementary Section S1.

3. Results and discussion 3.1 Local structural order and bond length distribution The arrangement of various ions in the final equilibrium configuration is shown in Fig. 1. The following set of effective ionic radii were used to compute the stacking distribution of ions: Si4+ (0.4 Å), Al3+ (0.535 Å), Ca2+ (1.0 Å), K+ (1.38 Å), Na+ (1.02 Å) and O2- (1.40 Å) 33. Ionic configurations representing the locations of charge compensation cations in the aluminosilicate network are shown in Fig. 1 (a). These results show that Ca2+, K+ and Na+ ions were present in the inter-ionic spaces available in the network and provided charge compensation to the nearest [AlO4]5- anion. In the stacking structure (Fig. 1 (b)), both Al3+ and Si4+ ions were present in the gap between closely stacked oxygen ions, whereas Ca2+, K+ and Na+ ions were present in the large cavities/voids between stacked oxygen atoms. The local structure of simulated aluminosilicates was found to be stable over the composition range and the relative proportions of both alkalis investigated. Simulated results on pair distribution functions (PDFs) and co-ordination numbers (CNs) for various ion pairs are shown in Fig. S1-S12 for K1N1, K3N1 and K1N3 systems. Detailed results for various alkali ratios have been provided in Tables S3-S6. The first (r1) and the second (r2) peak positions in these PDF curves represent the average values of the nearest and the next-nearest neighbor ions and and/or lengths of oxygen bonds. Average positions of first peak in the pair-distribution functions of various ions were unaffected by the increasing


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alkali concentrations or their relative proportions (Tables S3-S4). In the case of Si-O bonds, simulated nearest neighbor bond length (1.61 Å) was very close to the experimentally reported value (1.62 Å)

34

. The results for Al-O bonds (1.73 Å) also showed a good

agreement with experiment results (1.76 Å). While there were marginal differences in the second peak positions of ionic bonds of oxygen with other cations (Table S4), a mixed behaviour was observed in the second peak locations among various cations. The distance between Si-Si ions was found to increase with increasing concentration of alkalis from 5.18 Å to 5.37 Å. A similar behavior was observed in the individual presence of both alkalis in K1 and N1as well. In the case of Al-Ca ion pairs, the second neighbor distance was found to decrease from 6.08 Å to 4.01 Å in the simultaneous presence of both alkalis; whereas this distance had remained unchanged ~6 Å in the case of K1 and N1. Other cation pairs showed only minor fluctuations and no well-defined trend was observed. The relative proportion of alkalis did not have much influence on these PDFs. Fig. S13 shows that the presence of both alkalis and their relative proportions had a marginal influence on the CNs as well for various cation-oxygen bonds. These results indicate that apart from their influence on the second neighbor distances for Si-Si and Al-Ca bonds, the simultaneous presence of both alkalis had a negligible influence on PDFs and CNs of all other ionic bonds in the aluminosilicate structure. 3.2 Oxygen bonding networks As various cations are linked by oxygen atoms in the aluminosilicate network, the changes in oxygen bonding network are among the key factors influencing system properties. The bridging oxygens (BOs) connecting two polyhedron structures are known to


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increase the polymerization degree of the ionic network, the non-bridging oxygens (NBOs), connecting the polyhedron structure to a network modifier cation, can lower the degree of polymerization by breaking down the network. In addition to BOs and NBOs, tri-cluster oxygens (TOs), coordinated by three network-former cations, are an important oxygen type in aluminosilicates, especially those containing high levels of alumina. The presence of two types of oxygen tri-clusters in the final configuration: an oxygen tri-cluster O(Al, Al, Al) representing oxygen ions shared by three [AlO4]5+ tetrahedrons, and another oxygen tricluster O(Al, Al, Si) representing oxygen ions shared by two [AlO4]5+ tetrahedrons and one [SiO4]4+ tetrahedron 22, 27. Simulation results on the oxygen bonding networks in the present system are presented in Fig. 2. Horizontal dotted line in these plots represents various system characteristics in the absence of alkalis; results for pure Na2O (N1) or K2O(K1) have been included in these plots for the sake of comparison. The concentration of BOs was found to increase continuously in the presence of Na2O or K2O alone, albeit with a slower rate with increasing levels of alkalis in the system. The BO values showed a significant increase in the joint presence of both alkalis; these were much higher than the values reported for N1 or K1 alone. This change was observed for all total concentrations of both alkalis under investigation. The BO magnitudes showed a non-monotonic reduction with increasing proportions of K2O in the blend. A similar trend was observed for NBOs as well. A completely opposite trend was observed for the tri-cluster oxygens. The magnitudes of TOs was found to decrease continuously with a higher rate in the presence of K2O or Na2O alone with increasing levels of alkalis in the system. The TO values showed a significantly


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lower values reduction in the joint presence of both alkalis; these were much lower than the values reported for either N1 or K1. These results indicate that the simultaneous presence of both alkalis can have a significant influence on the oxygen bonding network in aluminosilicates. As the CaO/Al2O3 molar ratio was less than 1 in the system under investigation, Ca2+ ions cannot provide sufficient charge compensation to Al3+ ions to form stable Al-centered polyhedron. The addition of K2O and Na2O, K+ and Na+ can supply additional charge compensation which can increase the stability of network structure. An increase in the concentrations of bridging oxygens along with reductions in the levels of tricluster oxygens will decrease the concentrations of shared O2-, and provide a higher stability to the oxygen network. At a given total alkali concentration, the relative proportion of both alkalis (i.e. the K2O/Na2O ratio) was also found to have a strong influence on the magnitudes of all types of oxygens (BOs, NBOs and TOs). The magnitudes of both BOs and NBOs showed a continuous reduction with increasing proportions of K2O in the system, the corresponding TOs showed an opposite trend and a continuous increase with increasing levels of K2O. 3.3 Transport properties Individual self-diffusion coefficients for various ions were calculated during MD simulations, and the detailed results are presented in Fig. 3. The horizontal dashed line in all plots represents the values of diffusion coefficients in the absence of alkalis (K0N0); results for pure Na2O (N1) or K2O (K1) have been included in these plots. The selfdiffusion of Si ions was not much influenced by the presence of alkalis either by individual or joint presence; the magnitudes of diffusion coefficients for a range of alkali contents and


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relative proportions were found to fluctuate about the horizontal dashed line. The selfdiffusion of Al ions was found to decrease with increasing levels of alkalis present; increasing proportions of K2O also tended to slow down Al ion diffusion. A similar trend was observed for the diffusion of oxygen as well as Ca ions. These plots also show that there was a marginal influence on the self-diffusion coefficients of Al, O and Ca ions in the presence of Na2O alone or for high proportions of Na2O in the presence of K2O. While the diffusion of K and Na ions showed an increase with increasing concentrations of alkalis in the system; no well-defined trends could be identified for the simultaneous presence of both alkalis due of a fluctuating behavior. Fig. 4 shows plots of various self-diffusion coefficients with increasing proportions of K2O/Na2O (A-F) and Na2O/K2O (A’-F’) for a range of total alkali concentrations. These results are consistent with various diffusion coefficients presented in Fig. 3 and indicate a generally non-linear behavior. Simulation results on total diffusion coefficients of the system under a range of alkali concentrations and relative proportions of both alkalis are presented in Fig. 5. The results in the absence of alkalis (K0N0) and for only Na2O (N1) or K2O (K1) have also been included. The total diffusion coefficients were seen to increase with increasing concentrations of Na2O in the absence of K2O (N1); an opposite trend was observed for K2O in the absence of Na2O (K1). An intermediate but highly non-linear behavior was observed when both alkalis were present simultaneously; this aspect can be seen very clearly at the highest alkali concentration of 10%. There was a sharp reduction in the Dtotal in going from N1 (100% Na2O) to K1N3 (75% Na2O; 25%K2O); the reductions were much smaller for additional increases in the relative contents of K2O. Similarly, there was sharp increase in the Dtotal in going from K1 (100% K2O) to K3N1 (75% K2O; 25%Na2O); further increases were much


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smaller for additional increases in the relative contents of Na2O. These changes can be seen clearly in the line plots in Fig. 5b and 5c. Interestingly, there is small regime of Na2O/K2O ratios (between K1N1 and K2N1) for all alkali contents under investigation, where the magnitudes of Dtotal came very close to those of K0N0. The point of intersection between Dtotal in the presence of alkalis and the dashed curve (no alkalis) (Fig. 5d and 5e) was found to change from 1.9 to 1.55 K2O/Na2O ratios with total alkali content increasing from 4 to 8%; the corresponding values for Na2O/K2O ratios were determined to 0.27% and 0.7% with concentration of alkalis increasing from 2 to 8%. This result indicates that at these relative proportions of two alkalis, the influence of alkalis on the total diffusivity in molten aluminosilicates becomes almost negligible. This novel result from our simulations could prove to be of great significance in BF operations as it identifies relative proportions/ and concentrations of alkalis present in the system that may much have negligible influence on various aluminosilicate properties, including their viscosity. 3.4 Variation of Viscosity RNEMD simulation results (2223 K) on the viscosities of the system are presented in Fig. 6. Observed trends for viscosity (Fig. 6) were reciprocal to those reported for diffusivity in Fig. 5. The inverse relationship between diffusivity and viscosity can be seen clearly in Fig. 7. With the increasing presence of K2O in the alkali blend, there was a sharp initial increase in system viscosity followed by a much slower increase. Two distinct slopes and a changeover region can be seen clearly in Fig. 6b. With increasing relative concentration of Na2O in the blend (Fig. 6c), a sharp increase initially was followed by a


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sharp decrease and rather flat reduction at higher values of Na2O. All these plots clearly show a highly non-linear behavior and indicate that the effect of simultaneous presence of both alkalis cannot be determined by interpolation between the effects of individual alkalis. Next, we focus our attention to the region where various curves in Fig. 6b and 6c intersect the dashed curve indicative of slag viscosity in the absence of alkalis. In addition to the influence of relative proportion of both alkalis, the total concentration of alkalis was seen to play an important role (Fig. 6d and 6e). The point of intersection between the system viscosities in the presence of alkalis and the dashed curve (no alkalis) was found to change from 0.18 to 0.59 K2O/Na2O ratios with total alkali content increasing from 2 to 10%; the corresponding values for Na2O/K2O ratios were determined to increase from 1.9 to 2.85 with concentration of alkalis increasing from 6 to 10%. For these relative proportions and total amounts of alkalis present, there was no net effect of alkalis on the viscosities of aluminosilicates. 3.5 Implications for ash reactions in a blast furnace Based on previous research21,

22, 24

, the presence of K2O was found to increase the

viscosity (and reduce fluidity) of molten coke ash, and to slow down the flow of molten coke ash to the coke surface; the presence of Na2O was found to show an opposite trend. Migrated molten coke ash is likely to reside on the coke surface and create a barrier between the coke carbon and other reactants (gas, liquid metal etc.). However, coke ash with lower viscosity (and higher fluidity) would find it relatively easier to move away from the coke surface and to diffuse into the blast furnace slag as compared to molten ash with a higher viscosity. In an industrial blast furnace, both alkalis are expected to be present simultaneously, and influence the diffusivity of molten coke ash. Our work has shown that their joint effect on coke characteristics could be much smaller than predictions based on the influences of individual alkalis. When the K2O/Na2O ratios are located within the


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intersection zone (Fig. 6), their influence on the fluidity of molten ash is negligibly small. This result indicates that the relative proportions of K2O/Na2O need to be considered carefully while evaluating the overall influence of alkalis on blast furnace operation.

4. Conclusions In-depth MD simulations were carried out to investigate the influence of simultaneous presence of alkalis Na2O and K2O on the local structural order, oxygen bonding networks, transport properties, and the overall viscosity of the molten ash system: Al2O3-SiO2-CaOK2O-Na2O. The simulations were carried out at 2223K for a range of total alkali contents and the relative proportions of two alkalis. The main conclusions from this study are as follows: 1. The total concentration of both alkalis and their relative proportions had a marginal influence of the atomic configurations in the aluminosilicate network. The positions of first and second neighbor pair distribution functions and associated co-ordination numbers remained unaffected in the combined presence of both alkalis; these results were comparable to those obtained for individual alkalis. Only difference was in the second neighbor Al-Ca distance, which showed a rapid reduction from 6.08 Å to 4.01 Å in the simultaneous presence of both alkalis. 2. There was a significant influence of the simultaneous presence of both alkalis on the oxygen bonding network as compared to the individual presence of either Na2O or K2O. Both the BOs and NBOs showed a sharp increase and TOs showed a sharp reduction when both alkalis were present in the system. This trend, observed irrespective of the


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total alkali content or the relative proportions of two alkalis, points to an enhanced stability of the aluminosilicate structure. 3. There was a marginal influence of alkalis on the self-diffusion coefficients of various ions especially in the presence of Na2O alone or for high proportions of Na2O in the presence of K2O. Diffusion was generally found to slow down for the increased levels of K2O. Total diffusion coefficients showed a highly non-linear dependence on the relative proportions of two alkalis. The presence of small amounts of K2O along with Na2O resulted in a sharp reduction in the magnitude of Dtotal; the corresponding reductions at higher proportions of K2O were much smaller. Similar but opposite trends were observed upon additions of small amounts of Na2O to K2O. 4. System viscosity showed a non-linear behavior in the simultaneous presence of both alkalis. With increasing levels of K2O, the viscosity showed a sharp initial increase followed by an increase at a much slower rate; an opposite trend was observed for increasing proportions of Na2O in the system. 5. Simulation results of total diffusion coefficients as well as viscosity show that the combined influence of both alkalis on the system characteristics was significantly lower as compared to their individual presence (Na2O or K2O alone). There were several concentration regimes depending on total alkali content and/or their relative proportions, where the diffusivity and viscosity of the melt were very close to system characteristics in the absence of alkalis. 6. Our studies have shown that the overall impact of alkalis on the viscosity of molten ash was generally much smaller in the presence of both alkalis. This result was observed for a wide range of alkali concentrations as well as their relative proportions. As both


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alkalis are generally present simultaneously within an operating blast furnace, their influence on molten ash viscosity may be much smaller than predictions based on the effects of individual alkalis. This novel finding indicating a relatively smaller impact of alkalis on coke ash fluidity than generally perceived, is expected to be of immense significance to BF operations.

The relative proportions of K2O/Na2O need to be

considered carefully while evaluating the overall influence of alkalis on blast furnace operation.

Acknowledgements All these computations were performed on the GPC supercomputer at the SciNet HPC Consortium in the Compute/Calcul Canada National Computing Platform. SciNet is funded by the Canada Foundation for Innovation under the auspices of Compute Canada; the Government of Ontario; Ontario Research Fund - Research Excellence; and the University of Toronto. Kejiang Li and Jianliang Zhang acknowledge the financial support from the National Science Foundation of China (51774032).


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Table Captions Table 1 Miyake potential parameters used in this investigation 29

Figure Captions Fig. 1. The arrangement of ions in the final equilibrium configuration (red, cyan and green lines represents bonds with O, Al and Si respectively; purple, violet, orange balls represent K, Na and Ca cations respectively: (a) indicates the presence of charge compensation cations in the network bonds; and (b) represents the stacking state of constituent atoms. Effective ionic radii were used to represent the relative size of different ions. Fig. 2. The evolution of bridging, non-bridging and tri-cluster oxygen concentrations as a function of alkali content. Fig. 3. The dependence of self-diffusion coefficients of individual atoms on the total amounts of alkalis present Fig. 4. The evolution of self-diffusion coefficients of individual atoms as a function of K2O/Na2O and Na2O/K2O ratios. Fig. 5. The evolution of total self-diffusion coefficients of the system as a function of total alkalis present and their relative proportions. Fig. 6. The dependence of system viscosity as a function of concentrations and relative ratios of two alkalis present. Fig. 7. Correlation between total self-diffusion coefficient and the viscosity of the melt


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Table 1 Miyake potential parameters used in this investigation 29 c Ion

z

a (Å)

D

b (Å)

α r0 (Å)

Ion pair 1/2

3

1/2

(kJ Å /mol )

(kJ/mol)

(1/Å)

Si

1.92

0.5983

0.025

0.00

Si-O

63

2

1.47

Al

1.44

0.6758

0.030

0.00

Al-O

50.4

2

1.58

Ca

0.96

1.1425

0.042

30.74

Ca-O

21

2

2.2

K

0.48

1.5870

0.060

20.49

K-O

8

2

2.58

Na

0.48

1.0450

0.050

20.49

Na-O*

-

-

-

O

-0.96

1.7700

0.138

51.23

*Note: Na-O ion pair interaction in Morse term was not considered in the Miyake potential


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Fig. 1. The arrangement of ions in the final equilibrium configuration (red, cyan and green lines represents bonds with O, Al and Si respectively; purple, violet, orange balls represent K, Na and Ca cations respectively: (a) indicates the presence of charge compensation cations in the network bonds; and (b) represents the stacking state of constituent atoms. Effective ionic radii were used to represent the relative size of different ions.


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Fig. 2. The evolution of bridging, non-bridging and tri-cluster oxygen concentrations as a function of alkali content.


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Fig. 3. The dependence of self-diffusion coefficients of individual atoms on the total amounts of alkalis present.


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Fig. 4. The evolution of self-diffusion coefficients of individual atoms as a function of K2O/Na2O and Na2O/K2O ratios.


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Fig. 5. The evolution of total self-diffusion coefficients of the system as a function of total alkalis present and their relative proportions.


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Fig. 6. The dependence of system viscosity as a function of concentrations and relative ratios of two alkalis present.


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Fig. 7. Correlation between total self-diffusion coefficient and the viscosity of the melt.


Molecular Dynamics Investigation on Coke Ash Behavior in the High

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